This invention relates to a bottom ash submerged scraper conveyor and in particular to a method and apparatus for automatically controlling the speed of a bottom ash submerged scraper conveyor drag chain as a function of the ash loading thereon.
A bottom ash submerged scraper conveyor is used for the continuous removal of ash from a water impounded trough located beneath a furnace. The submerged scraper conveyor is operated continuously because the water impounded trough has relatively little storage capacity when compared to a conventional water impounded hopper beneath a furnace. The ash which falls from the furnace is quenched by the water and settles to the bottom of the trough. The submerged scraper conveyor drag chain consists of flights located between two parallel lengths of chain that scrape along the lower surface of the water impounded trough and transport the settled ash to and up the dewatering slope at the end of the submerged scraper conveyor. At the top of the dewatering slope, the drag chain passes around a sprocket, reversing direction and dumping the conveyed ash into a discharge. The drag chain returns through a compartment beneath the water impounded trough to the opposite end of the scraper conveyor. Again the drag chain passes around a sprocket reversing direction to repeat the cycle of passing through the water impounded trough and transporting ash to and up the dewatering slope.
The drive unit for the submerged scraper conveyor rotates a headshaft at the top of the dewatering slope where the flights and chain pass around a sprocket reversing direction. The drive unit may be any appropriate variable speed drive including a hydraulic drive or a variable speed electric motor. The speed at which the submerged scraper conveyor drag chain and flights travel ranges from two feet per minute to eighteen feet per minute with the normal speed less then ten feet per minute to reduce wear and power consumption. The speed of the submerged scraper conveyor drag chain, although adjustable, has been achieved by manually selecting an appropriate speed and setting the drive unit to achieve the selected speed. The selected speed would necessarily have to be great enough to remove the bottom ash generated under the worst conditions anticipated until the selected speed setting was subsequently reset. Since the submerged scraper conveyor wears while in operation, even if unloaded, the submerged scraper conveyor would operate unnecessarily fast causing needless wear.
Therefore a need exists for an automatic speed control for a submerged scraper conveyor. It is an object of the present invention to provide an automatic speed control for a submerged scraper conveyor that would sense the required ash removal capacity of the submerged scraper conveyor and adjust the speed of the submerged scraper conveyor drag chain in response to the required ash removal capacity. The automatic speed control would sense the torque on the headshaft and adjust the speed of the submerged scraper conveyor drag chain so that the torque on the headshaft is controlled to the design value. Maintaining the design torque on the headshaft assures that the capacity of the submerged scraper conveyor to remove bottom ash is commensurate with the rate at which the bottom ash is generated. In this manner, automatic speed control of the submerged scraper conveyor drag chain maintains the availability of the required bottom ash removal capacity for all furnace operating conditions.